Porous moldings made from thermoplastics are known. They are used extensively in various areas of technology, for example for gas and liquid filtration at elevated and normal temperatures (filter cartridges). They are also used for the dispersal of gases in liquids, as insulating materials for refrigeration equipment, as thermal protection, vibration elements, cushioning materials, boat bumpers, electrolytic membranes in batteries, air cushions, and in the textile and packing industries, among others. Numerous processes have been developed for the production of porous moldings from thermoplastics. Their differences take into account the properties of the starting materials and the requirements made of the products.
In a process described in DE-C-1 176 850, solid, porous articles are produced by sintering mixtures of two polyolefins of different molecular weights. The polyolefins differ by at least 5.degree. C. in their softening ranges and by at least 5000 in their viscosimetric molecular weights. The mixing ratio between low-molecular-weight and high-molecular-weight polyolefins is from 1:1.5 to 10 parts by weight. The mixture is introduced into molds and heated to the temperature necessary for sintering.
A modification of the above procedure is described in DE-C-1 232 743. This publication teaches the production of solid, porous articles by sintering mixtures of at least two polyolefins which differ in their viscosimetric molecular weight ranges. The mixtures contain from 40% to 80% by weight of the low-molecular-weight polyolefins, and the molecular weight range of the high-molecular-weight polyolefins is at least three times that of the low-molecular-weight polyolefins.
DE-C-1 255 298 discloses a process for the production of sintered, porous articles from pulverulent polymers of aliphatic olefins. The polymer powder is allowed to drop loosely onto a heated plate, and the individual particles are fritted to one another by constant heating. The process is continued by placing finely divided starting material in layers on the fritted base layer, fritting them to one another in this way and thereby building up a porous article.
The above-described processes start from thermoplastics which melt or at least soften at elevated temperature. If it is certain that the individual particles soften only at the surface, they coalesce to form a stable solid through which pass channels which ensure it is permeable to fluid media. A variant of this process uses mixtures of at least two polyolefins, of which at least one softens on heating and acts as a binder for the other.
In contrast to the low-molecular-weight thermoplastics described above, high-molecular-weight polymers do not melt without degradation on heating, but instead are converted into a viscoelastic state. The polymer particles therefore cannot bind strongly to one another at the points of contact by surface melting and subsequent solidification of the melt. The production of porous moldings from such thermoplastics therefore requires special processes which differ from those used for the processing of plastics of relatively low-molecular-weight.
The high-molecular-weight plastics which do not melt without decomposition on heating, but instead take on viscoelastic properties, include ultrahigh-molecular-weight polyethylene (PE-UHMW). This is linear polyethylene of extremely high melt viscosity which results from the low pressure process. Its intrinsic viscosity is at least about 1000 ml/g, corresponding to weight average molecular weight of at least about 1.times.10.sup.-6. The conversion of the intrinsic viscosity into molecular weights is according to the Margolis equation, and the method used for the determination is described, for example, in CZ-Chemische Technik 4, 1974, page 129 ff.
PE-UHMW is distinguished by a number of physical properties which give it a variety of potential applications. It has high resistance to wear, a low coefficient of friction against other materials, excellent toughness, and high heat distortion resistance. In addition, it is notably resistant to numerous chemicals. These particular mechanical, thermal and chemical properties enable ultrahigh-molecular weight polyethylene to be used as a high-grade specialty material in a wide variety of areas, including porous moldings.
Various processes are known for the conversion of high-molecular-weight polyethylene into porous moldings. Thus, according to DE-C-1 241 599, pulverulent polymers having molecular weights of from 250,000 to greater than 500,000 and having broad melting ranges are used for this purpose. The starting material is heated in molds, giving solid, porous, uniformly sintered polyethylene materials.
Microporous membranes made from polyethylene having a weight average molecular weight of at least 5.times.10.sup.5 and in particular from 1.times.10.sup.6 to 10.times.10.sup.6 are obtained, according to EP-B-160 551, by dissolving the polymer in a solvent and warming the solution. A gel film is formed from the solution, and the solvent is removed to a solvent content of 10% to 80% by weight. The film is subsequently warmed, stretched and free from residual solvent.
U.S. Pat. No. 4,925,880 relates to porous moldings made from ultrahigh-molecular-weight polyethylene having a molecular weight in the range from about 1.times.10.sup.6 to about 6.times.10.sup.6 and a polyethylene wax whose molecular weight is from about 1000 to 20,000. The pulverulent, heterogeneous mixture is heated in a compression mold under pressure until the wax melts. At the same time, the PE-UHMW particles soften and expand. Adjacent particles thus come into contact and bond to one another. The material is rapidly cooled, and the porous article is removed from the mold.
The preparation of porous materials from mixtures of PE-UHMW and another polyolefin, for example a polyethylene or a polypropylene, is also described in JP-A-86-283634 (cf. C.A. 106, Abstract 157477a). As an example, the preparation of a porous material comprising 40% PE-UHMW and polypropylene by sintering for 15 minutes at 200.degree. C. is described. Particular mention is made of the resistance to chemicals, the mechanical strength, and the gas permeability.
A porous material consisting exclusively of ultrahigh-molecular-weight polyethylene is described in WO-A-92/08757. It is built up from individual polyethylene particles which have been bonded to one another to form pores. It is obtained by injection molding of plasticized PE-UHMW at a shear rate of at least 5.times.10.sup.4 sec.sup.-1 under such conditions that the quotient of the weight of the molding and its volume does not exceed 0.7 g/cm.sup.3. In a preferred embodiment, a pore-forming additive, for example sodium chloride, is added to the plasticized PE-UHMW. The additive is removed from the molding with the aid of a solvent therefor which is inert to the polyethylene.
The known processes are not free from disadvantages. They require, for example, the use of mixtures of PE-UHMW and low-molecular-weight polyolefins or the addition of additives and thus impair certain properties of the ultrahigh-molecular-weight polymer. In other cases, the processes are not universally applicable, but are restricted to the production of moldings having certain dimensions.
It is, therefore, among the objects of the Invention to provide a process which yields porous moldings of ultrahigh-molecular-weight polyethylene. The process should, if possible, be universally applicable, should not require mixtures of ultrahigh and low-molecular-weight polyethylenes or the use of additives, and in addition, should be capable of being carried out using conventional means.